Discovery of C5H+ and detection of C3H+ in TMC-1 with the QUIJOTE line survey

TL;DR

This study reports the first detection of C5H+ and C3H+ ions in the cold starless core TMC-1 using the QUIJOTE survey, providing insights into their abundance and chemical behavior.

Contribution

The paper presents the first identification of C5H+ and C3H+ ions in TMC-1, including precise frequency measurements and abundance ratios, advancing understanding of interstellar ion chemistry.

Findings

C5H+ detected with four harmonic lines in TMC-1.

First detection of C3H+ in a cold starless core.

C5H+/C3H+ abundance ratio is approximately 3.7.

Abstract

We report the discovery of the C5H+ cation toward TMC-1 with the QUIJOTE line survey. Four lines from J=7-6 up to J=10-9 have been identified in perfect harmonic frequency relation that can be fit with B=2411.94397 +/-0.00055 MHz and D=138+/-3 Hz. The standard deviation of the fit is 4.4 kHz. After discarding potential candidates, C5H- among them, we conclude that the carrier is C5H+, for which accurate ab initio calculations provide B=2410.3 MHz. We also report for the first time in a cold starless core the detection of the C3H+ cation. The column densities we derive for C5H+ and C3H+ are (8.8+/-0.5)e10 cm-2 and (2.4+/-0.2)e10 cm-2, respectively. Hence, the C5H+/C3H+ abundance ratio is 3.7+/-0.5. The fact that C5H+ is more abundant than C3H+ is well explained by dedicated chemical models and is due to the slow reactivity of C5H+ with H2, while C3H+ reacts with H2.